冷空气让面团变得有点硬。

The cold air is ___stiffening the dough a bit.

学校正在加强对迟到学生的处罚。

The school is ___stiffening punishments for students who are late.

随着比赛的临近，他的斗志越来越坚定。

His determination was ___stiffening as the competition approached.